The present invention relates generally to the production of substrates used in the manufacture of thin-film photovoltaic cells wherein a thin film of solar energy converting semiconducting material is supported on the substrate, and more particularly to the production of such substrates during which the substrates are provided with surface texturing for promoting light capturing and reflecting properties of the substrates.
Increasing concerns for the environment and the substantial reliance on exhaustible energy resources for supplying the energy needs of the world have been largely responsible for the increased efforts in developing alternative energy sources. For example, considerable research is being conducted in developing systems for converting solar energy into useful energy forms such as electricity. Notably, the conversion of solar energy directly to electrical energy in an environmentally nonpolluting manner has been achieved through the use of photovoltaics or solar cells.
Generally, the production of electrical energy in photovoltaic cells is provided by utilizing amorphous or crystalline semiconducting materials which possess solid state characteristics capable of efficiently converting solar energy or photons into electrical energy. Such semiconducting materials include silicon doped with suitable impurities such as aluminum, copper, chromium, phosphorous and boron to provide N-type and/or P-type junctions and to a lesser extent because of relative cost, semiconducting materials such as cadmium sulfide, cadmium telluride, amorphous silicon-hydrogen, germanium, and gallium arsenide. Of the types of photovoltaic cells as presently known, the so-called thin-film photovoltaic cells which utilize a relatively thin layer or film of a suitable semiconducting material of a thickness less than about 200 microns deposited on a supporting substrate are presently generating the most interest. It has been observed that available thin-film photovoltaic cells already possess solar energy conversion efficiencies at least as great as the well known and relatively expensive solar cells formed of a single crystal and that developments in thin-film photovoltaic technology are expected to provide even greater improvements in cost efficiency and solar energy conversion efficiency.
In the fabrication of thin-film photovoltaic cells, the solar energy converting semiconducting material is deposited onto a surface region of a substrate by a suitable deposition technique such as chemical vapor deposition as a film with a thickness in the range of about 50 to 200 micrometers. The selection of the substrate material and providing the substrate with desirable physical properties are of significant importance in the manufacture of efficient thin-film photovoltaic cells at economically acceptable costs. The substrate is required to be made of a material which forms a non-rectifying contact of relatively low electrical resistance with the deposited film of semiconducting material. The substrate is also required to provide an adequate structural support for the film to maintain the integrity thereof and provide a tenacious physical bond with the deposited film. Further, the substrate must possess a coefficient of thermal expansion substantially corresponding to that of the deposited film in order to minimize deleterious stresses which may occur therebetween during exposure of the thin-film photovoltaic cell to temperature variations.
Satisfactory substrates for thin-film photovoltaic cell applications have been formed of sintered ceramic materials such as alumina, silica, silicon carbide, mullite, cordierite, zirconia or mixtures thereof and possess a coefficient of thermal expansion sufficiently close to that of the deposited thin films of the semiconducting materials so as to provide a structural support for the thin films when the thin-film photovoltaic cells are subjected to a wide range of temperatures. Also, these ceramic materials provide an adequate physical bond with the deposited thin film and possess electrical properties required of the substrate. The use of a substrate formed of relatively inexpensive materials is an important consideration from the standpoint of cost efficiency in the use of photovoltaic cells as a viable alternative energy source.
When generating electricity with thin-film photovoltaic cells, a substantial percentage of the photons pass through the thin film without being utilized in the electrical energy producing process occurring within the film. Thus, in order to increase the solar conversion efficiency of thin-film photovoltaic cells, a mechanism must be provided for increasing light trapping within the cells so that photons passing through the thin film can be collected and reflected back into the thin film for use in the energy conversion process. To this end, it has been found that the enhancement of the light trapping and collecting of the thin-film photovoltaics can be successfully provided by using thin film dielectric reflectors or by providing the surface of the substrate supporting the thin film with light reflecting properties.
Previous efforts utilized in providing the substrate with light reflecting properties included forming the substrate in such a manner as to have a roughened film supporting surface to collect and reflect the photons passing through the thin film back into the film. This "texturizing" of the substrate has been provided by forming the substrate from a sintered blend of ceramic particulates provided by a relatively large size fraction of particulates and a relatively small size fraction of particulates. However, it was found that the formation of the substrate in such a manner had a considerable impact on the structural integrity of the substrate since such substrates were relatively friable and thus lacked the desired structural strength needed for supporting the thin film of semiconducting material, especially when employed in relatively thin cross sections such as required for maintaining a cost efficient production of thin-film photovoltaic cells. Also, some texturing of the substrate surface has been achieved by mechanically roughening the surface of the sintered ceramic materials in order to increase the light-trapping properties of the substrate. This mechanical roughening of a surface on sintered ceramic substrates is relatively expensive, substantially nonreproducible and also considerably increases the cost of the substrates employed in the manufacture of thin-film photovoltaic cells.